The present invention relates to a soft magnetic material useful as a core, a yoke or the like installed in various types of magnetic sensors such as electric power steering, fuel injection systems for vehicles and A.C magnetic circuits such as solenoid valves.
An A.C. magnetic circuit is built into an electromagnetic induction sensor, e.g. a differential coil magnetic sensor or a flow sensor, or a mechanical quantity sensor, e.g. a magnetostrictive torque sensor or a phase-differentiated torque sensor. Another type of a sensor, which uses an exciting coil as a detection coil, is already known. A core and a yoke as parts of such the A.C. magnetic circuit are made of soft magnetic material such as pure iron, Si steel, soft ferrite or permalloy.
Displacement of an object or a torque is detected as a slight change in impedance or voltage of the detection coil originated in displacement of the object by applying A.C. to the exciting coil so as to produce an alternating field.
The demand for improved measurement accuracy becomes stronger and stronger along with the development of magnetic sensors. Since a reduction of noises during detection of output voltage is necessary in order to improve measurement accuracy, a high-frequency (e.g. 100 Hz-5 kHz) electric current with a sine or rectangular wave is applied to an exciting coil.
However, eddy current loss of electromagnetic soft iron (SUYP), which has been commonly used as soft magnetic material, increases in proportion to a frequency increase of the applied magnetic field, resulting in decrease of magnetic induction necessary for a sufficient output voltage. Si steel is advantageous in less eddy current loss due to its high electric resistivity compared with electromagnetic soft ion, but Si content necessarily increases in order to suppress reduction of magnetic induction in an alternating field with frequency not less than 1 kHz. Although an increase of Si content effectively enlarges the electric resistivity, Si steel is hardened and degraded in press-workability.
Corrosion resistance is also one of the required properties of soft magnetic material, which is expected to be used in a special environment. But, electromagnetic soft iron and Si steel are poor in corrosion resistance. Corrosion resistance may be improved by formation of a Ni or chromate treatment layer, but such plating increases the cost of a product. The plating unfavorably degrades magnetic properties and also causes a deviation of magnetic properties due to irregularity in thickness of the plating layer.
Permalloy, especially permalloy C, is a material having an excellent A.C. magnetic property with high electric resistivity, but is very expensive. Soft ferrite is high in electric resistivity with less reduction of magnetic induction in a high-frequency zone not less than 10 kHz compared with metal material, but its magnetic flux density is less than that of metal material in a frequency zone not more than 5 kHz on the contrary.
Fe-Cr alloy has been heretofore used as yokes for a stepping motor due to is high electric resistivity, good corrosion resistance and cheapness compared with permalloy. However, in the case where conventional Fe-Cr alloy is used as a part in a magnetic circuit such as a magnetic sensor operated in a low-magnetic field less than 10 Oe with frequency of 100 Hz-5 kHz, sufficient output voltage necessary for accurate measurement is not obtained at a detecting terminal.
The present invention aims at providing a new cheap Fe-Cr soft magnetic material, excellent in properties as a magnetic sensor operated in a high-frequency low-magnetic field as well as corrosion resistance.
The newly proposed Fe-Cr soft magnetic material has electric resistivity not less than 50 xcexcxcexa9xc2x7cm and a metallurgical structure composed of ferritic grains at a surface ratio not less than 95% with precipitates of 1 xcexcm or less in particle size at a ratio less than 6xc3x97105/mm2 in number.
The Fe-Cr soft magnetic material preferably has the composition consisting of C up to 0.05 mass %, N up to 0.05 mass %, Si up to 3.0 mass %, Mn up to 1.0 mass %, Ni up to 1.0 mass %, P up to 0.04 mass %, S up to 0.01 mass %, 5.0-20.0 mass % Cr, Al up to 4.0 mass %, 0-3 mass % Mo, 0-0.5 mass % Ti and the balance being Fe except inevitable impurities, under the conditions of (1) and (2).
4.3xc3x97%Cr+19.1xc3x97%Si+15.1xc3x97%Al+2.5xc3x97%Moxe2x89xa740.2xe2x80x83xe2x80x83(1) 
64xc3x97%Si+35xc3x97%Cr+480xc3x97%Ti+25xc3x97%Mo+490xc3x97%Alxe2x89xa7221xc3x97%C+247xc3x97%N+40xc3x97%Mn+80xc3x97%Ni+460xe2x80x83xe2x80x83(2) 
The soft magnetic material is manufactured by providing a Fe-Cr alloy having the specified composition, forming the Fe-Cr alloy to an objective shape, and heat-treating the formed Fe-Cr alloy in a zone between 900xc2x0 C. and a temperature T (xc2x0C.) defined by the formula (3) in a vacuum or reducing atmosphere. The wording xe2x80x9csoft magnetic materialxe2x80x9d means a material, which is not shaped to a magnetic part yet, in various forms of sheets, rods or wires in response to its application.                                                                         T                ⁡                                  (                                      xc2x0                    ⁢                                          xe2x80x83                                        ⁢                                          C                      .                                                        )                                            =                              xe2x80x83                            ⁢                              (                                                      64                    xc3x97                    %                    ⁢                                          xe2x80x83                                        ⁢                    Si                                    +                                      35                    xc3x97                    %                    ⁢                    Cr                                    +                                      480                    xc3x97                    %                    ⁢                                          xe2x80x83                                        ⁢                    Ti                                    +                                      490                    xc3x97                    %                    ⁢                                                                  xe2x80x83                                            ⁢                                              xe2x80x83                                                              ⁢                    Al                                    +                                                                                                                                          xe2x80x83                                ⁢                                                      25                    xc3x97                    %                    ⁢                                          xe2x80x83                                        ⁢                    Mo                                    +                  480                                )                            -                              (                                                      221                    xc3x97                    %                    ⁢                                          xe2x80x83                                        ⁢                    C                                    +                                      247                    xc3x97                    %                    ⁢                    N                                    +                                      40                    xc3x97                                                                                                                                          xe2x80x83                            ⁢                                                %                  ⁢                                      xe2x80x83                                    ⁢                  Mn                                +                                  80                  xc3x97                  %                  ⁢                                      xe2x80x83                                    ⁢                  Ni                                            )                                                          (        3        )            